The present invention relates to a centering device usable in particular for centering in a well a drilling assembly composed of a drill bit and drill collars above same.
The problem of centering drilling assemblies has thus far been solved by placing in the bottom assembly a number of "stabilizers" or centering devices with straight or helical blades which effectively ensure centering of the drill pipes to some extent but at the cost of permanent friction against the borehole wall, since they are rotationally integral with the assembly. In soft rock, this friction results in a widening of the borehole at the level of the stabilizers, and this widening eventually destroys the desired centering function. Once a stabilizer has dug out an accommodation, there is now nothing to prevent the pipes from rubbing against the wall which has not yet been widened.
In vertical drilling, the widening may not be very substantial: the transverse force applied to the pipes is zero in theory, if one overlooks the force of buckling of the drill collars as well as the dynamic effects due to misalignment, slight though it may be. In general, there is no preferred direction for these potential lateral forces and they may be considered to cancel each other out overall, as far as their effect on deflection goes.
In deflected wells, the widening can no longer be overlooked since the rock supports part of the weight of the assembly and even all of the weight in a horizontal well. As a result, the borehole gradually becomes oval in shape and tends to deviate, generally to the right in view of the usual rotational direction of the string of drill pipes due to the reaction to the rolling of the assembly on the wall and wear of the pipes, which may be fairly rapid in abrasive rock.
In the case of sufficiently sharp curvature, the body of the pipe reams out its own accommodation which the larger-diameter equipment located below (stabilizers, tool, etc.) will not be able to pass when raised to the surface again. This is the phenomenon known as "key-seating" which also occurs in the upper part of the assembly where it is the tool joints which become jammed.
It may be seen from this description of phenomena familiar to drillers that the friction of the pipe string on the borehole walls is difficult to control and is often the source of costly problems. It consumes a substantial part of the mechanical power transmitted to the pipe string by the turntable (on the order of 75% at 2000 meters in a borehole with a 121/4" or 31 cm diameter inclined 20.degree. to the vertical), making it extremely difficult to maintain the desired trajectory.
The contradictory solutions supplied to this problem by drilling equipment designers are a reflection of the difficulties encountered for example, it is recommended that the number of stabilizers be increased or also recommended that reamers be employed with the idea of rapidly eroding the borehole walls to reach a certain "profile of equilibrium" more rapidly. Additionally, the stabilizer blades are polished in order not to over-widen the borehole and to reduce the torque loss.
One original solution is to have the stabilizer not rotate with the pipes. The blades are generally rubber shoes attached to a jacket of the same material, in which the assembly may rotate freely. Lubrication is provided by the mud (and debris). Lengthwise translation of the jacket along the body is possible between two annular stops, the lower stop being provided with teeth designed to block rotation if need be (in the event of overdrilling or jamming when the equipment is raised). It seems however that the use of these tools is not very widespread, probably because of their short service lives.
The same principle is used for certain key-seat reamers where the jacket is made of metal and fitted with tough blades, usually helical. The upper stop is then provided with a cam allowing lengthwise hammering.
In, for example, U.S. Pat. No. 2,815,930 a key-seat reamer is made rotationally integral with the pipe string when axial displacement occurs between the blades of the reamer and the pipe string due to an obstruction. This drive is provided by mutual meshing of teeth rotationally integral with the reamer blades and teeth rotationally integral with the pipe string.
Such a device ensures that rotation comes to a complete stop before the teeth are engaged, failing which it would be exposed to severe mechanical stresses which are always detrimental.
The present invention proposes a centering device which does not in general rotate with the pipes, hence ensures effective centering, and yet provides for the possibility of reaming by driving blades rotationally but limiting this possibility to occasions when it is truly necessary, i.e. when the drilling assembly has become jammed in the lengthwise direction. The device according to the invention avoids the drawbacks mentioned above.
Thus, the present invention relates to a device having at least one centering device relative to which said assembly can rotate about its axis. This device is characterized in particular by having means for driving said device rotationally, said means comprising a friction clutch.
This clutch can in particular be a disk clutch, a cone clutch, or a drum clutch.
This clutch may have several disks or cones, some of which will be rotationally integral with said centering device and others with said assembly, these disks or cones overlapping each other.
The device according to the invention may comprise elastic means for positioning the various disks relative to each other.
The device according to the invention may comprise jaw clutch means.
Likewise, the device according to the invention may comprise means for controlling other means for gradual initiation of rotation, said control means being activated above a certain value, the threshold of the difference between the axial stress to which said assembly is subjected and the stress to which said device is subjected.
These control means may comprise return means such as springs.
The control means of said drive means may be assisted by a pressurized fluid.
These control means may be hydraulic and mechanical means combined.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purposes of illustration only, several embodiments in accordance with the present invention.